Lucina is an import model, so it's just a replacement of the .pac and .pcs files.
@Bass forever
You can't use Lucina over Roy because the files are too big. If you want to get it to work, you're going to need to resize and reduce the quality of the textures to get it to work. But that would make it look ugly.
Roy's PM and Brawl limit is around 750kb for the files and anything larger than that, you'd need to swap with Marth.
They share the same hacks but Roy is a completely custom character and can only handle less.
On the topic of SSB4, I'd like Masked Lucina to be an alternate, sharing the same palette as Marth rather than Princess Lucina since you wouldn't want to spoil the game.
Plus it's as easy as putting a mask over Marth's face and making Lucina slightly thinner than Marth. If they do this, I have no doube we'll also get the Parallel Falchion for the alternate as well.
Lucina's taunt could be her pose in Ferox Arena, her turning away and taking the mask off, and a voiced taunt akin to her portrait cut ins during battle in FE Awakening.